The present invention relates to a laser generation method and device adopting the same, and more particularly, to an efficient method for the generation of blue and green lasers and a device adopting the same.
In a laser generating mechanism, when an intense pumping light irradiates an atom having electron layers with base energy levels, an electron is excited into a higher level due to the energy of the pumping light. Such an excited electron is not in a stable state and thus returns to the base level. The energy absorbed when the electron returns is emitted as a light energy. Here, the wavelength of the emitted light is similar to or longer than that of the pumping light that initially excites the electron.
A laser device generating a light having a shortwavelength, for example, blue and green laser devices, has a wide application and many studies are currently underway in various fields. From an optical viewpoint, shorter wavelength light can be focused more narrowly. Based on such characteristics of light, as the wavelength of light being used for optical recording becomes shorter, the recording density of information is increased. Therefore, far more data can be recorded in the limited region of the existing media currently in use. For this purpose, the light having a short wavelength and good interference is desirable. Therefore, laser diodes are mainly used as a light source.
Shortwave laser diodes have been developed in various types. However, a semiconductor laser diode is in wide use since optimum miniaturization is required.
The currently used semiconductor laser device being in the infrared region generates light having a long wavelength. As a result, there is a limit to the obtainable increase in recording density with respect to a given medium. To overcome such a limitation, the development of a device for generating lased light having a shorter wavelength is an urgent matter. Putting such blue and green laser devices into practical use is time consuming, and there have been many studies on this matter. However, no case shows that the semiconductor laser device oscillates for long periods of time at room temperatures, which is caused by the unique characteristics of the semiconductor material itself.
To obtain the blue and green lasers, second harmonic generating devices using a laser as a pumping energy having a long wavelength, for example, 800 nm to 980 nm, are developed. It should be noted that the blue and green lasers are not generated from the laser device itself, rather the second harmonic generation is one in which the wavelength of the generated infrared light is halved by employing a non-linear optical material within an generation section optically confined by two mirrors. An apparatus for second harmonic generation has a complicated structure and thus limits miniaturization. For example, in a second harmonic generating device, a non-linear bifringent crystalline used for obtaining the second harmonic changes its characteristics depending on temperature variations. Therefore, a multitude of peripheral components for a temperature control are required.
Meanwhile, as another alternative for obtaining a laser having a short wavelength, a frequency up-converted laser device for secondly exciting an electron at the state where the electron is excited by a light having a long wavelength so as to generate a wavelength having a short frequency, is employed.
The frequency up-converted laser device utilizes a second excited state absorption of an electron caused by a pumping light. The principle can be explained as follows.
As shown in FIG. 1, when an atom is irradiated by an external pumping light, electrons are excited from a base level E.sub.0 to a first energy level E.sub.1. In addition, the once-excited electrons are excited again to a second energy level E.sub.2, being higher than first energy level E.sub.1, before the electron returns back to the low energy state of level E.sub.0. The twice-excited electron directly returns from second energy level E.sub.2 to base level E.sub.0. As described above, when the electron excited twice returns to base level E.sub.0, light is generated. At this time, if an appropriate resonance condition is given to the generated light, a laser is generated. Here, the wavelength is shorter than that of the light being initially incident due to the two excitements of the electron.
Under such a principle, the first energy level difference and the second energy level difference of the electron should be the same when the pumping light is converted from a long wavelength to a short wavelength. The light obtained in such a process is a laser having a short wavelength, i.e., half that of the pumping light having a long wavelength.
To apply such a principle, as shown in FIG. 1, an atom having the same difference between the first energy level and base level and the second energy level and first level, has to be found. As an electron stays longer at the first excited state (level E.sub.1), a probability of moving the electron to the second energy state becomes higher. Therefore, it is necessary to make the electron stay longer at first energy level E.sub.1.
A frequency up-converted laser device employing such a principle was realized in 1990, and an erbium-doped fluoride fiber laser was disclosed in 1991. Thus, the generation of a laser having a wavelength of 546 nm by employing an infrared ray laser diode having a wavelength of 800 nm as a pumping source results in success. Starting from this, there has been a study on the blue and green laser devices employing a fluoride fiber. For the case of a frequency up-converted laser device employing the second excited state absorption, many developments have been achieved from the standpoint of structure or efficiency, as described in U.S. Pat. No. 5,299,215. However, a fluorine which is difficult to be formed into a fiber-type structure is used as a basic medium material in order to make an electron stay longer at a first excitement level. As for the fluorine, it is difficult to use a modified chemical vapor deposition which is employed as a general method for manufacturing a fiber. Now, as a general method for manufacturing a fiber using the fluorine, there is a built-in casting process which includes a melting process for making the base material of a fiber. Thus, controlling thickness is difficult and doping a predetermined amount of oxygen is impossible. As a result, it is hard to correctly control the diameter of a core which greatly affects the generation mode of a laser, or to miniaturize the core. Further, it is hard to obtain a laser having a single mode by using elements employing such basic medium material due to structural restrictions described above.